Central focus solar energy collectors include circular concentrators and heliostats. Typically a circular concentrator has a segmented or continuous parabolic-dish mirror or fresnel lens to concentrate the direct solar output on a receiver which is located at the focal point of the parabolic dish or fresnel lens. Heliostats (as in this invention) consist of a plurality of flat (or nearly flat) mirrors which are subjected to two-axis control to cause reflections of the direct rays of the sun from all mirrors to converge on a receiver. Concentrating solar energy in these ways, central focus solar energy systems typically operate at from 1000.degree. F. to 2500.degree. F. which is much higher than the nominal 150.degree. F. provided by flat-plate collectors or the 500.degree. F. characteristic of linear focus (parabolic trough) collectors. Heat is normally transferred from the receiver (or absorber) using a suitable operating fluid for storage and/or use in a thermal-to-electric conversion system. Concentrator photovoltaic cells, which directly convert some of the concentrated direct rays of the sun into electricity, may also be installed on the central focus system receiver. Using concentrated solar energy in this way, a given electrical power demand may be met with fewer expensive photovoltaic cells than would otherwise be required with no concentration. Typically silicon concentrator cells operate at from 12% to 23% efficiency in converting solar energy to electricity at concentrations of from 25 to 100 suns. AlGaAs cells have a reported potential efficiency of up to 25% at from 50 to 2000 suns. In the photovoltaic cell configuration, air or an operating fluid flowing thru the receiver, cools the photovoltaic cells for higher solar-to-electrical conversion efficiencies and transfers heat to a thermal storage unit.
Thus advantages of central focus collectors over other solar energy systems include the following:
Higher operating temperatures than possible with flat-plate or linear focus collectors permitting: PA1 Optional use of concentrator (as apposed to conventional) photovoltaic cells at high sun concentrations, thereby reducing the cost of generating electricity thru the direct conversion of solar energy to electricity. PA1 A plurality of flat (or nearly flat) mirrors which are pivotably mounted and mechanically coupled so that all mirrors may be simultaneously and equally rotated about either one or both of two axes of rotation. PA1 Mirror support fixtures, which are pivotably mounted in fixed frameworks or other structural members to rotate about what are called mirror "principal axes" of rotation in this invention. Principal axes for all mirrors are parallel to one another and parallel to the earth's spin axis. PA1 A second axis of rotation for each mirror which is termed "secondary axis" in this application. These secondary axes are perpendicular to, intersect and rotate about principal axes. PA1 A means for initially adjusting or focusing each mirror so that during system operation, reflections of the direct rays of the sun from all mirrors converge at a common focus. PA1 Electromagnetic or other drive mechanisms for rotating all mirrors about their principal axes in accordance with diurnal (i.e., time-of-day) changes in the apparent position of the sun. PA1 Electromagnetic or other drive mechanisms for rotating all mirrors about their secondary axes to cope with seasonal changes in the sun's declination angle.
Higher thermal-to-electric conversion efficiencies PA2 Smaller thermal energy storage subsystems PA2 More extensive industrial use of solar energy